Media captionMPs accuse the UK Border Agency of giving up on tracing missing asylum seeker and immigration applicants while under pressure to clear backlogs.

MPs have accused the UK Border Agency (UKBA) of losing track of a population of asylum seekers and migrants equivalent to the size of Cambridge.

The Home Affairs Select Committee said 124,000 asylum seeker and immigration cases had been "dumped" in an archive, with the agency giving up on them.

The term "controlled archive" was used to try to hide the fact that it was a list of lost applicants, said the MPs.

Ministers and the opposition blamed each other for the reported failings.

The committee looked at what had happened to a backlog of approximately 450,000 unresolved asylum cases inside the UKBA.

John Reid, Home Secretary in 2006, had pledged that the "legacy" cases would be dealt with by the end of 2011.

The backlog included thousands of cases which were duplicates or the paperwork was irrelevant because the case had been resolved in some other way.

But that left tens of thousands of individuals with whom the UKBA had lost contact. Each of these cases has been classified in the so-called controlled archive and is checked against watch lists for six months. If there is still no chance of tracing the individual, the case is then marked as "concluded".

We are making real progress tackling the archive to trace these individualsDamian Green, Immigration Minister

In November 2010, there were 18,000 cases in this controlled archive. But the committee said it had risen in 10 months to 124,000, roughly the population of Cambridge.

It includes about 98,000 asylum seekers who cannot be found and 26,000 immigration cases, involving those who have overstayed visas or have been refused an extension of leave, such as students.

The MPs said: "While we appreciate the difficulties involved in tracing people with whom the UKBA have lost contact, usually for a period of several years, it is clear that the controlled archive has become a dumping ground for cases on which the agency has given up.

"The controlled archive has increased significantly as the deadlines for the legacy backlog and the migration case review have approached.

"We also object to the term 'controlled archive'. It is another instance of a bureaucratic term which hides the true nature of a government department's activity and is designed to deflect attention away from it.

"The controlled archive would be more appropriately referred to as an archive of lost applicants."

The committee said it saw no reason why the controlled archive should grow, given that the legacy clearance exercise had now been completed.

Committee chairman Keith Vaz criticised bonuses he said were being paid to border officials while the agency was "continuing to fail".

He told the BBC: "It's very significant that at a time when there are stringent cuts in government expenditure, when the agency is simply not performing, that senior officials at the Home Office, in the UK Border Agency, have paid themselves between £5,000 and £10,000 each as far as bonuses are concerned.

"This is not acceptable and it really does need to be resolved. That means a change of Cabinet Office guidance to say that there must be no more bonuses until all the problems at the UK Border Agency have been resolved."

Chris Bryant, the shadow immigration minister, said: "These numbers betray a shocking failure at the heart of this Tory-led Government.

"The government's chaotic approach to immigration enforcement has meant 100,000 people have been quietly dumped into the 'untraceable' file and borders staff have given up on dealing with them."

But Immigration Minister Damian Green blamed Labour.

"The immigration system we inherited was chaotic and this government is working to fix the mistakes of the past by making better decisions, ensuring cases are properly tracked, improving intelligence and speeding up removals.

"I am determined to deal with the historic asylum cases left by the last government and we are making real progress tackling the archive to trace these individuals.

"Since taking office we have nearly doubled the number of failed asylum seekers removed within one year of their application and now conclude cases quicker, with almost two-thirds decided within a month."